1. Field of the Invention
The present invention relates to an image forming apparatus. More particularly, the present invention relates to an improved paper detecting apparatus for detecting the presence or absence of printing paper in a feed cassette of the image forming apparatus, and an image forming apparatus having the same.
2. Description of the Related Art
In general, a color image forming apparatus with a printing function, such as a laser printer, a copier, a facsimile, or a multifunction peripheral, has the following configuration. With reference to FIG. 1, an image forming apparatus has a plurality of color-specific photosensitive media 12, 13, 14 and 15 inside the main body 100 of the image forming apparatus. A plurality of laser scanning units (LSU) 22, 23, 24 and 25 irradiate a laser beam to the respective photosensitive media to form electrostatic latent images on the photosensitive media. A plurality of developing units 32, 33, 34 and 35 supply developing solution to the respective photosensitive media with the electrostatic latent images to form a visual image. The visual images formed on the respective photosensitive media are transferred and superimposed upon one another on an intermediate transferring medium 50 by a plurality of first transferring units 42, 43, 44 and 45. The final images formed on the intermediate transferring medium 50 are transferred to a printing paper P supplied by a second transferring roller 60. The final images transferred to the printing paper P are fixed by a fixing unit 70 that applies high-temperature heat and pressure to the images. The printing paper is then discharged outside the image forming apparatus. In the lower part of the main body 100 of the image forming apparatus, a feed cassette 110 supplies sheets of printing paper inside the main body 100 of the image forming apparatus. Furthermore, the image forming apparatus has a paper detecting sensor or the like to sense whether printing paper is loaded in the feed cassette. If there is no printing paper, the paper detecting sensor sends a signal to a controller of the image forming apparatus, thereby stopping the printing operation and alarming users.
FIG. 2 shows a conventional paper detecting system. The paper detecting system comprises a paper plate 120 having one end fixedly installed to the feed cassette 110, a biasing spring 130 that elastically biases the paper plate 120 to a pickup position, and a pickup roller 140 picking up the printing paper loaded on the paper plate 120 to deliver the printing paper inside the main body 100 of the image forming apparatus. A paper guide lever 150 regulates the distance between the top and bottom of the sheets of printing paper stacked in the paper plate 120, while sliding along a guide groove (not shown) formed on the bottom surface of the feed cassette 110.
A paper detecting sensor 160 is installed in the main body 100 of the image forming apparatus to be located at the upper front end of the paper plate 120, thus detecting whether printing paper is stacked on the paper plate 120 and sending a detected signal. The paper plate 120 is formed with a paper plate groove at a position corresponding to the paper detecting sensor 160.
Thus, when the last sheet of printing paper remains on the paper plate 120, the paper detecting sensor 160 detects the printing paper by receiving light 161 reflected from the paper. When the last sheet loaded on the paper plate 120 is printed out and the rear end of the printing paper moves out of a detecting area of the paper detecting sensor 160, the paper detecting sensor 160 does not sense the light because the light 162 passes through the paper plate groove (not shown) formed on the paper plate 120, and thus concludes that there is no printing paper.
In a tandem-type color image forming apparatuses, the laser scanning units 22 to 25 irradiate a laser beam on the photosensitive media 12 to 15 before a sheet of paper in the feed cassette is picked up by the pickup roller 140. This is to increase the printing output rate of the system.
Thus, when continuous printing output is carried out and the number of sheets to be printed is more than the number of sheets of printing paper loaded in the feed cassette, the lack of printing paper is detected after the laser beam has already irradiated the photosensitive media and development has begun. Accordingly, developing agent is consumed unnecessarily. Also, since images on the photosensitive media 12 to 15 and the intermediate transferring medium 50 have to be cleaned, the cleaning load is increased. Further, the second transferring roller 60 is contaminated; hence, when outputting the next sheet after loading new sheets of printing paper, the backside of the sheet is contaminated.
To address these problems, a paper detecting system installed with the paper detecting sensor 160 at an upper part of the rear end of the paper plate 120 has been developed. This paper detecting system alleviates some of the described problems, but introduces other problems. In particular, the length of printing paper that can be used is limited because the paper detecting sensor 160 is fixed.
To address this problem, the paper detecting sensor 160 may be installed on a paper guide lever 150. If this is done, however, because the feed cassette is separated from the main body of the image forming apparatus, a separate connector needs to exchange signals between the paper detecting sensor and the controller installed in the main body of the image forming apparatus. Furthermore, when mounting the feed cassette to the image forming apparatus, the connector has to be connected to the controller. Accordingly, the paper detecting system as mentioned above is inconvenient for a user.
Accordingly, there is a need for an improved detector for detecting the presence of paper in a paper feed cassette for an image forming apparatus, and an image forming apparatus having the same.